This invention is concerned with a liquid ring pump. Such a pump essentially comprises a bladed rotor mounted within an eccentric casing into which ring liquid or seal liquid is introduced and, under the centrifugal force produced by rotation of the rotor, is caused to form a ring following the interior contour of the casing. The blades of the rotor and the inner surface of the ring define working chambers or buckets which are alternately brought into communication with inlet and outlet ports and into which a gas is admitted during a suction stroke and from which the gas is expelled as the bucket volume contracts.
Different porting techniques are adopted for admitting gas to the buckets and for allowing the exit of gas from the buckets. For example, a so-called center port pump is known in which the gas enters and leaves the buckets radially. A side port pump is known in which the gas enters and leaves the pump axially. Combinations of those two types of pump are also known in which one of the inlet and outlet ports communicates radially with the buckets while the other communicates axially with the buckets.
Additionally, the pumps may have casings which define a single lobe such that there is one operational cycle per revolution of the rotor or the casing may define multiple lobes, there being as many cycles per revolution as there are lobes. It will be recognized from the following description that the subject matter herein is applicable to the various kinds of pumps.
The invention is specifically concerned with pumps of the two stage kind that is to say is concerned with pumps comprising a first pumping stage, the outlet of which is connected to the inlet of a second pumping stage, the second pumping stage being of lesser capacity than the first pumping stage. One pump of this general kind is described in German Pat. No. 823,170.
It is well known that in two stage pumps the first stage is constructed with several times the volumetric displacement capacity of the second stage. At low vacuum levels the first stage pump discharges a gas volume rate greater than that which can be handled by the second stage, i.e. a gas volume rate in excess of the capacity of the second stage. If the second stage has a full liquid ring and the interstage, i.e. the connection between the first and second stages, is not otherwise vented, the excess volume of gas passed from the first stage over that which can be accommodated by the second stage, is trapped which results in high pressures between the first and second stages. This situation results in such performance problems as a high power requirement, reduced first stage capacity and surging, i.e. unstable pumping action. Attempts to solve this problem have included the provision of a bypass check valve to vent the excess of the pump's capacity, the check valve opening whenever the volume of gas moved by the first stage exceeds that which can be handled by the second stage. This solution is generally adequate but, of course, relies upon a mechanical device for its effectiveness and the valve is subject to breakdown or malfunction.
An alternative technique is described in the aforementioned German patent specification. In that patent there is provided an unloader hole in the second stage which is effective to bleed seal liquid from that stage to decrease the thickness of the liquid ring and necessarily, therefore, to increase the capability of the second stage to permit the gas to pass directly from its inlet to its discharge. This method also relies on a valve to regulate the amount of liquid discharge from the second stage and, of course, one configuration or valve setting providing for a specific discharge at a nominal operating condition does not give good performance characteristics at different speeds and seal flow rates. Further, this latter method is one which does not provide tolerance to seal flow variations and to rotor speed variations.